Heatsealing method



Aug. 25, 1953 N. LANGER 2,550,183

HEATSEALING METHOD original Filed July l5, 195o INVENTOR.

face of the machine.

Patented Aug. 25, 1953 UNITED STATES PATENT OFFICE 2,650,183 HEATSEALING METHOD Nicholas Langer, New York, N. Y.

Original application July 15, 1950, Serial No.

Divided and this application August 7, 1951, Serial No. 240,653

4 Claims.

November 6, 1951.

As disclosed in my Patent No, 2,460,460, heatsealing machines of the thermal impulse type essentially comprise a pair of pressure members or bars and a reciprocating mechanism therefor whereby pressure may be applied upon two or more layers of thermoplastic film to be heatsealed interposed therebetween. Examples of the commercially most important thermoplastic films are Pliolm (rubber hydrochloride), Vinylite (a copolymer of vinyl chloride and vinyl acetate), Polythene (polyethylene), and Saran (vinylidene chloride). A heater element in the form of a thin and narrow strip of a metal of high specific resistance, such as a suitable nickelchromium alloy known in the trade as Nichrome, is mounted on the face of at least one of said bars, constituting the operating or sealing sur- Sealing pulses of electric current may be passed through the said heater element under the control of a switching mecha-i nism, the operation of which is coordinated to that of the reciprocating mechanism of the bars.

During the operation of the machine, operation of the switching mechanism is initiated substantially when the pressure members arrive into their pressure-applying position. A short pulse of current is then passed through the heater element, which, as a result of its low heat capacity, is heated to heatsealing temperature in a small fraction of a second. The heat thus produced is immediately transferred by surface contact to the region of the thermoplastic layers compressed between the bars, causing heatsealing thereof. A short period thereafter, the heat produced by the pulse of current is `dissipated and the seal is cooled and consolidated under pressure whereby a strong and sound seal is obtained.

Heatsealing machines of the thermal impulse type provide important advantages over the commonly used heatsealing machines in which the sealing member is continuously maintained at a constant sealing temperature. Due to the fact that the thermal impulse principle permits the sealed region to cool and to consolidate under pressure, the quality ofthe seal is greatly improved. Also, the thermal impulse principle makes it possible to readily seal films of materials, which cannot `be sealed on an industrial scale with sealing members continuously maintained at constant sealing temperatures, such as particularly Polythene and Saran.

Although the heatsealing machines of the thermal impulse type provided excellent results consolidation of the seal. I'his diihculty interfered with the desired high operating speeds, which to obtain is the present trend in the entire packaging industry.

I have now discovered that the outstanding problem may be solved in a remarkably simple manner.

It is an object of the present invention to improve heatsealing methods of the thermal impulse type.

associated structure after each sealing operation or cycle.

It is also within `the contemplation of the theinvention will become apparent from the fol- VWidth of the said is made, since Nichrome,

' heat conductivity. Y considerably wider Fig. 1 is a side elevational view, somewhat fragmentary in character and having parts in section, of a preferred embodiment. of the invention;

Fig. 2 is a section taken on line and Fig. 3 .illustrates a modified heater element suitable for the purposes of the invention.

Referring now more particularly to the drawing, reference numeral II) generally denotes a sealing device embodying essentially comprises a metal formed of metal of high heat conductivity,`

aluminum. ItV is preferred metal to whichV aj soldered or braced 2-2 of Fig. l;

basel vor bar AI IY to use a joint can be readily made. I2 may be provided in the bar, extending along its length for passing a coolant fluid, for ,example water, therethroug Upon the top surface of bar II, there is provided a first layer I3 of heat-resistant insulating material, such as Teflon (polymerized tetrafluoroethylene) ,Y Kel-F (polymerized tetraiiuorochloro-ethylene) with silicone resin, and the like. A thin and narrow heatervstrip I4 of metal or alloy of high specific resistivity is mounted or tensioned on the rstrinsulating layer I3 `and constitutes a heater element of low heat capacity for the bar. amples of suitable high-resistivity materials for the heater strip I4 are Nichrome (a nickel base alloycontaining 1l22%A chromium and smaller amountsA of silicon and manganese), Inconel (a Y nickel base heat-V and oxidation-resistant alloy chromium, 6% iron,V

with approximately 13%' small amounts of manganese, silicon and copper, and Nilvar (a high-nickeliron alloy with about 36% of nickel Yshowing extremely low coefficient of temperature expansion). A second layer I5 of heat-resistantv insulating material is placed on top of the heater strip I4. This second layer of insulationv `I5 may be formed of the same material as that of the rst layerof insulation I3. It will be noted that the twolayers of insulation I3 and lI5 vhave approximately thesame width'asthat of thetop surface of base or bar II Vwhile the he ter strip I4, which is centrally located between the two insulatinglayers, is of considerably lesser width, Vsuch as 1/3 or 1/4 of the layers, Furthermorarthe lower insulating layer |03 is of substantial thickness as compared tothe thickness of the upper insulating layer I5 which is made as thin as possibleand is compatible with the requirements of electrical insulation to be provided between the underlying heater strip or element I4 and the overlying sealing strip IB. As la result', an intimate heat exchange relation will be established between the heater strip I4 andthe sealing strip IB Vthrough the'v minute thickness of the interposed insulating layer'I5. 'Y Sealing strip I6 is formed of metal of relatively low heat conductivity, such as of the same metals or alloys of which the heater strip I Inconel or Nilvar, in addition to having low electrical conductivity, also possess'the`characteristic of relatively low The sealing strip is made than the width of bar II and its lateral marginal portions are bent over the top edges of the bar and are pressed against the sides thereof, resulting yin an inverted U-shaped cross section. The edges of the sealing 'strip are securedto the bar in any suitable manner, for example by bracing or soft soldering, as inthe invention whichYV extended beyond the ends of bar or base II so f that conductors dicated at I1. The same result may be obtained by means of metal clamping strips I8 pressed against the side faces of the bar by screws IS. The top surface 20 of sealing strip I6 constitutes the operating or sealing face of the bar.

Upon assembling the sealing device, sealing strip I6 is tensioned aroundthe top face of bar Il and xes the underlying members I3, I4 and I in their properly assembled relative position. The two ends 2l and 22 of the heater strip I4 are leads 23 and 24 may be re- Of course, more or spectively Asecured thereto.

elaborate terminal arrangements may be re- Y A 'channel uor duct sorted to, if desired. Nipples 25 and 25 are in- Y serted into the ends of coolant channel I2 whereby the said channel may be readily connected into a flow of coolant fluid by means of rubber tubing.

The sealing device or bar Il) is mounted for cooperation with Ya conventional pressure bar 2l, the lower face of which is preferably provided with a layer or facing '28 of elastic material, for example, silicone rubber. The sealing and pres sure bars are arranged for relative reciprocation by means of any suitable conventional manuallyoperable or motor-driven mechanism.

From the foregoing description, the operation of the sealing device of the invention will be readily understood by those skilled in the art. When it is desired to make a seal, layers 29 and 3i) of thermoplastic film are interposed between sealing bar II and'pressu're bar 2l and the bars are displaced toward one another to apply presd sure upon the regions of the layers to be heatsealed. A short pulse of sealing current of suit.' able intensity is then passed through heater strip I4 by connecting its ends 2|, 22 to a source of electric current (not shown) through lead wires 23 and ,24. This pulse of current will heat up the said strip to :a high temperature practically instantaneously. As.V the lower insulating layer I3 is considerably heavierthan the upper insulating layer I5,`most of the heat generated in the heater strip I4 will travel upwardly and by conduction throughtheminute thickness of insulating layer I5 will rapidly heat up a center strip I6 in'aregion roughly portion of sealing corresponding to Ythe width of the underlying heater strip I4 and'denoted by reference numeral 3l Vin Fig. 2. Heatsealing of the compressed layers29, 30 of the thermoplastic film will now occur in the heated regionV 3I 'of the sealing face 20. l

A short period thereafter, the sealing'heat is dissipated and theseal formed is allowed to cool and to consolidate'while the pressure upon theY seal is still mai tained. Finally,Y theA pressure is released and the sealed layers are withdrawn. 'i

Any residual heatremaining in I6 is rapidly conducted away into bar sealing strip I I formedof metal of high heat conductivity. This is due `strip is very small, the heat withdrawn into the temperature vto such moderate readily dissipated by radiation Ystructure ldescribed is bar will raise its extent as will be or convection. 1 Thus, the

entirely satisfactory for.heatsealing machinesV wherein no extreme speeds of operation are required. .I- loweven the rapidityo heat withdrawal from the sealed region of film and from the sealing face of the bar may be further accelerated to any practically necessary extent by passing water, air or some other suitable coolant fluid through channel I2 of bar II. This is accomplished, for example, by introducing the coolant fluid into channel I2 through inlet nipple 25 and discharging such fluid through cutlet nipple 26. As an alternative, a plurality of cooling Vplates or ribs may be mounted on or may be integrally formed with bar II and a blast of air may be directed against the same.

It is to be observed that the heatsealing method of the invention provides various important advantages. The most important of these advantages may be listed as follows:

l. While the sealing strip I6, particularly its operative face 20 is in intimate heat exchange relation with the heater strip I4 through the minute thickness of the interposed insulating layer I5, it is electrically disconnected therefrom by the same layer. Therefore, no current will flow through the sealing strip itself and the current consumption of the entire unit is relatively low.

2. The center region 3I of sealing strip I 6 is intensely and intermittently heated by the underlying heater strip I4 during each heating or sealing cycle while its marginal regions are efficiently and continuously cooled byhaving the lateral edges thereof soldered, brazed, clamped `or otherwise secured to corresponding portions of bar II. Thus, heating or sealing pressure is applied to the thermoplastic layers in the center of their compressed region while cooling pressure is applied to said layers in the remainder of such region. It will be further noted that the 'said heated and cooled areas in the sealing face are defined in one and the same perfectly smooth and plane surface 20 so that no extrusion or cutting of the sealed layers can occur in the boundary lines defining the said heated and cooled areas. This, of course, is due to the relatively low heat conductivity of the material of the sealing strip which permits maintaining a sharp temperature gradient between different transverse regions thereof.

3. In general, the width of the seal produced roughly corresponds to the width of the heater strip I4 which is preferably about 1/3 or 1A; of the width of the sealing face I8. However, the width of the seal may be adjusted and increased by increasing the intensity or the duration of the sealing pulse of electric current. This will be readily understood if it is considered that the sealing heat produced in heater strip I4 will first heat up the directly overlying portion 3| of sealing face 20. However, in case the sealing pulse of current is sufficiently long or intense, the heat will travel in the plane of the sealing face in both transverse directions a short distance and this will result in widening of sealing region 3|.

4. As the sealing face of the unit is completely smooth and as there is no extrusion of the thermoplastic nlm possible during the sealing operation, sticking of the sealed nlm to the sealing face is practically completely absen 5. Removal of the residual heat from the sealing face of the unit and from the sealed regions of the thermoplastic layers is rapid and extremely effective. Therefore, the time required for a complete heating and cooling cycle may be considerably reduced.

6. The sealing bar is extremely rugged both mechanically and electrically since the sealing strip I6 is anchored along the full length of both of its longitudinal edges and is mechanically supported by the underlying generally non-compressible structural elements. On the other hand, the mechanically vweak heater strip I4 is fully protected against injury by the sealing strip I6 which completely envelops the same. Thus, the useful life of the sealing device is quite long.

7. The sealing device or bar of the Ainvention is very simple .in structure and may be readily manufactured on a practical and industrial scale in any desired length or shape at a low cost.

In order that thoseskilled in the art may have a better understanding of the invention, the following illustrative example may be given:

In `a practical sealing device or bar embodying the invention, bar or base I'I was formed of a brass rod of oblong cross section, having a width of 0.5" and a height of 1".. The length of the bar was 12", but 'this dimension, of course, is determined by the length of the desired seal. Insulating layer I 3 was formed of a strip of glass fiber fabric impregnated with silicone resin, having a width of 0.5 and a thickness of 0.01". Heater strip I4 was formed of a Nilvar strip, 0.1" wide and 0.00.2" thick. Insulating layer I5 was formed of a strip of glass fiber fabric impregnated with silicone resin, having a width of 0.5" and a thickness of 0.003.

and the overlying sealing strip I6, while electrically insulating 'the two elements from each other, its thickness may be considerably less than lSealing strip I6 was formed of a Nilvar strip having a Width of 1.5 before bending it into U-shape and a thickness of 0.002.

The advantage of using Nilvar for both the heater and the sealing strips resides in the extremely low coefficient of lin-ear heat expansion of this material.

Fig. 3 illustrates a modified heater element which may be substituted for heater element or strip I4. This modified heater element comprises a narrow strip 32 of heat-resistant insulating material, such as mica, glass-fiber fabric impregnated with silicone resin, asbestos, or the like. Windings of wire 33 of high specific resistivity, such as Nichrome, are wound around strip 32 throughout the length thereof, the ends of said wire being soldered or otherwise secured` to rivets or eyelets 34 and 35 provided at the two ends of the strip. Lead wires 36 and 3l are attached to members 34 and 35', respectively, and serve for connecting the heater element to a source of electric current.

The advantage of the modied heater element just described resides in the fact that by proper selection of the resistance wire diameter and of ance of the heater strip I4 is generally quite low so that it is in many cases necessary to energize such a strip through a step-down trans makes it possible tov eliminate the transformer which results in considerable saving.

Although the present invention has been disclosed in connection with a few preferred embodiments thereof, variations and modifications may be resorted to by those` skilled in the art without departing from the principles of the invention. I consider all of these variations and modifications to be within the true spirit and scope of the present invention, as disclosed in the foregoing description Vand. defined by the appended claims.

I claim:

1. The method of heatsealing layers of thermoplastic film which comprises pressing one face of a strip of sheet metal against said layers, generating a pulse of sealing heat at the other face of said strip and in a region narrower than said strip to transfer such pulse of heat by conduction through the thickness of the strip to said layers of film and to cause heatsealing thereof in said region, and withdrawing residual heat from said strip by conduction in the plane thereof through a direct metallic path including the lateral marginal portions of the strip into a body of relatively high heat capacity. l

2. The method of heatsealing layers of thermoplastic lm which comprises compressing the layers between a pair of normally cool surfaces at least one of which is formed by one face of a strip of sheet metal, practically'instantaneously generating a pulse of sealing heat at the other face of said strip and in a region narrower than said strip to transfer such pulse of heat by conduction through the thickness of the strip to said layers of film thereby to cause heatsealing thereof in said region, continuously withdrawing residual heat from said strip by conduction in the plane thereof through a direct metallic path including the lateral marginal portions of the strip into a body of relatively high heat capacity thereby to cool the sealed region, andthen releasing the pressure.

3. The method of heatsealing layers of thermoplastic film which comprises providing a direct metallic heat conducting path between the lateral marginal portions of a stripof sheet Vmetal and a body of relatively Vhigh heat capacity maintained at al constant low temperature, pressing a surface of said strip lagainst said layers', producing a thermal impulse at the other surface of said strip and in a central region narrower than said strip to transfer such impulses by conduction through the thickness of the strip to said layers of film thereby to cause heatsealing thereof in said region, the said conductively cooled marginal portions of the strip being effective in withdrawing residual heat from the sealed region of the layers, and then releasing said pressure after the sealed region has cooled and consolidated.

4. The method of heatsealing layers of thermoplastic film which-comprises continuously withdrawing heat at the lateral marginal portions of a strip of sheet metal through a direct metallic path into a body of relatively high heat capacity, pressing one face ofsaid strip against said layers, dissipating a predetermined amount of electrical energy and thereby generating a thermal impulse adjacent to the other face of said strip and in a region central to but narrower thanl the strip, said thermal impulse being transferred by conduction through the thickness of the strip to said layers of nlm and causing heatsealing thereof in said region, the heat Withdrawn at the marginal portions of the strip being eiective in cooling said sealed region by conduction of residual heat in the plane of the strip, and then releasing the pressure after the sealed region has cooled and consolidated.

Y NICHOLAS LANGER.

References Cited inthe le of this patent UNITED STATES PATENTS Number Name Date 2,460,460 Langer Feb. 1, 1949 2,574,094 Fener et al. Nov. 6, 1951 2,574,095 Langer Nov. 6, 1951, 

1. THE METHOD OF HEATSEALING LAYERS OF THERMOPLASTIC FILM WHICH COMPRISES PRESSING ONE FACE OF A STRIP OF SHEET METAL AGAINST SAID LAYERS, GENERATING A PULSE OF SEALING HEAT AT THE OTHER FACE OF SAID STRIP AND IN A REGION NARROWER THAN SAID STRIP TO TRANSFER SUCH PULSE OF HEAT BY CONDUCTION THROUGH THE THICKNESS OF THE STRIP TO SAID LAYERS OF FILM AND TO CAUSE HEATSEALING THEREOF IN SAID REGION, AND WITHDRAWING RESIDUAL HEAT FROM SAID STRIP BY CONDUCTION IN THE PLANE THEREOF THROUGH A DIRECT METALLIC PATH INCLUDING THE LATERAL MARGINAL PORTIONS OF THE STRIP INTO A BODY OF RELATIVELY HIGH HEAT CAPACITY. 